1. Field of the Invention
This invention relates to a polyurethane prepolymer and the resulting sponge that can be made from the prepolymer which has the general appearance and wiping characteristics of a cellulose sponge, with minimal swelling when wet and with the property of not curling during drying.
2. Description of Previously Published Art
It has long been a goal of the sponge industry to substitute polyurethane foam for cellulose foam for use in wiping sponges or in sponge-based composites such as scrubbing pads or bathmats. In general, polyurethane foams of the previous art which are hydrophilic enough to be useful, tend to swell greatly when they are wet such as on the order of more than 75-100% by volume. They dry slowly and nonuniformly, causing curling or cockling as they dry. This property is objectionable for household sponge use. It has been proposed to use polyurethane foams as the substrate for flocked wall coverings. However, in view of the curling and cockling properties of these foams and the resulting buckling that occurs, these foams are unfit, to be used as the substrate for flocked wall coverings.
In U.S. Pat. No. 3,806,474, a hydrophilic polyester urethane foam is disclosed. However, this foam does not wet out rapidly and it does not wipe so as to leave a clean surface.
In U.S. Pat. No. 4,160,076, a simulated natural sponge based on hydrophilic polyurethane is disclosed. However, even though this simulates a natural sponge, it still has the problem of polyurethane sponges that it swells greatly when it is wet.
Conventional polyurethane foams have been made from MDI which is methylene-bis (phenyl isocyanate). These foams are rigid or semi-rigid because MDI imparts crystallinity. They are generally hydrophobic because MDI itself is hydrophobic. In British Pat. No. 874,430, flexible polyurethane foams are produced by reaction of polyether polyols with at least two hydroxyl groups and a polyisocyanate mixture consisting of diarylmethane diisocyanates and 5 to 10 percent by weight of a polyisocyanate having a functionality greater than 2 in the presence of a small amount of water. A catalyst can be used in optional embodiments. These foams have the disadvantages that they are not hydrophilic and are not made with sufficient quantities of water to allow transport of large amounts of fibers, fillers, antiseptics, or other water-soluble or water-dispersible components into the foams and in the case of the optional catalyst there can remain catalyst residue which is not desirable.
In U.S. Pat. No. 4,237,240, flexible MDI-based foams with high load-bearing and high energy-absorption capacity are made by reaction of diphenylmethane diisocyanates with polyester polyols or mixtures of polyester polyols and polyether polyols with a polyester polyol content of at least 60 percent by weight of the polyol mixture, and small amounts of water. As set forth in the claims, a catalyst is employed. These foams have the same drawbacks as those of the above-described British Pat. No. 874,430 including the undesirable catalyst residues in the foam and in addition they require the use of the more expensive polyester polyols.
In British Pat. No. 1,209,058, flexible hydrophilic polyurethane foams can be made by reacting a polyisocyanate with polyether polyols which contain at least 10% by weight of a block copolymer of ethylene oxide capped with propylene oxide to obtain hydrolytic stability. The method requires using at least one divalent tin salt of a fatty acid and/or at least one tertiary amine as a catalyst. The foam products made by this method, while being hydrophilic, have the drawback of being made with only small amounts of water. Moreover, there is no teaching of the use of MDI, which is hydrophobic to make hydrophilic foam products and the resulting foam will contain undesirable catalyst residues.